1. Field of the Invention
The present invention relates to a lighting fixture having a fluorescent lamp and a heat conductive member for cooling the fluorescent lamp.
2. Description of the Related Art
Today, several lighting fixtures including a low-pressure mercury vapor discharge fluorescent lamp are available for producing light. The fluorescent lamp shows improved luminous flux if the temperature of the fluorescent lamp is controlled. In particular, the cool spot of the surface of an air tight or hermetic vessel of the fluorescent lamp should achieve a predetermined temperature. A vent hole in the main part of the lighting fixture helps to control the temperature of the fluorescent lamp is well known. The known vent holes may not, however, provide adequate cooling. For example, a lighting fixture for recessed mounting in a ceiling has an undersurface cover. The underside cover may raise the temperature in the main part of the lighting fixture, because the underside cover reflects light and heat. If the temperature of the cool spot of the fluorescent lamp may thus exceed the optimum temperature, and the luminous flux deteriorates.
Japanese Utility Model Laid Open No. 1-68610 discloses a fluorescent lamp having a U shaped transparent hermetic vessel and a lamp base mounted one end of the vessel. The tip portion of the fluorescent lamp projects beyond a reflective board in the inner side of the main part of the lighting fixture. The top portion of the lighting fixture has a vent hole, and heat reflected from the reflective board can radiate around the tip of the fluorescent lamp and out the vent hole.
In the device of Japanese Utility Model Laid Open No. 1-68610, the vessel of the fluorescent lamp tends to receive damage from the hole in the reflective board. Moreover, the portion of the fluorescent lamp located outside the reflective board does not contribute to luminescence.
Japanese Patent Laid Open No. 5-225819 also discloses a fluorescent lamp having a U shaped transparent hermetic vessel and a lamp base mounted one end of the vessel. In the inner side of the main part of the lighting fixture, metallic supports hold the tip portion of the fluorescent lamp, and the composition of the supports allows the main part of the lighting fixture to conduct heat generated by the fluorescent lamp.
Japanese Patent Laid Open No. 4-303510 discloses a thin type of lighting fixture. In this type of lamp, the inside diameter of the fluorescent lamp is thin. The fluorescent lamp is operating by a high frequency lighting circuit. The fluorescent lamp has the inside diameter 23-26 mm and is attached in the lighting fixture. This fluorescent lamp is referred to as a T8 type so called a compact lamp. The lighting fixture has a means to conduct heat because the ends of the fluorescent lamp are thermally connected to the reflector. Furthermore, a vent hole, in a position corresponding to the end of the transparent hermetic vessel at the ceiling side of the lighting fixture, allows heat from the fluorescent lamp conducted by the reflector to radiate. Compared with a lamp having large diameter, the surface temperature of T8 type tends to become high. Because heat cannot escape from the main part of the lighting fixture, the surface temperature of the hermetic vessel rises. The mercury pressure in the fluorescent lamp thus becomes too high and self-absorption of light by the mercury increases. The fluorescent lamp brightness thus deteriorates compared with the highest value, and the luminous efficacy decreases. Furthermore, because the wall load on the transparent hermetic vessel increases when the diameter of the inner transparent hermetic vessel is made small like T5 type so called the compact lamp, the temperature of the transparent hermetic vessel becomes even higher.
The fluorescent lamp is connected to the reflector to conduct heat in Japanese Patent Laid Open No. 5-225829 and Japanese Patent Laid Open No. 4-303510. However, when the reflector becomes hot, the cool spot of the fluorescent lamp is not cooled effectively.
The wall load in a fluorescent lamp of T5 type or T8 type is relatively large. The compact lamp has a high lamp surface temperature, compared with the fluorescent lamp of so called T10 type, so heat of the compact lamp cannot effectively escape from the main part of the lighting fixture. The mercury pressure thus becomes too high and self-absorption of light by the mercury increases.
Because the wall load of the fluorescent lamp increases when the diameter of the transparent hermetic vessel is made small like T5 type, the temperature of the transparent hermetic vessel becomes increasingly high. Then leak current becomes a problem in achieving high frequency lighting of the fluorescent lamp. The fluorescent lamp with a thin diameter like the T5 type has a high rate of leak current compared with a fluorescent lamp with a thick diameter and the same consumption of electric power. The reason is that the part to which the fluorescent lamp voltage went up, and lamp current decrease since the diameter is thin. Therefore, when the lighting fixture has a means are thermally connected to the fluorescent lamp, leak current must be considered.